Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to laser gyroscopes such as solid state fiber optic gyroscopes and ring laser gyroscopes and, more particularly, to an interconnection in a housing for such gyroscopes for maintaining the magnetic shielding properties of the housing.
2. Description of Related Art and Other Considerations
The present invention is generally applicable to laser gyroscopes, such as solid state fiber optic gyroscopes and ring laser gyroscopes. Both provide paths for directing travel of coherent laser light. In the former, optical fibers in a housing provide the paths. In the latter, travel is provided within a lasing chamber which comprises a laser housing having bores and mirrors positioned where the bores change direction. One problem relating to such laser gyroscopes involves polarization changes due to the Faraday effect, also termed Faraday rotation. This problem can arise when the magnetic shielding about the housing is inadequate, that is, magnetic flux leaks into to housing, with the result that magnetic lines contaminate or interfere with the traverse of the coherent light about its path.
Because the present invention was conceived to address problems relating to fiber optic gyroscopes, the following exposition will be directed thereto; however, it is to be understood that the present invention is as applicable to ring laser gyroscopes and similar instrumentation utilizing coherent radiation and the specific problems solved by the invention described herein.
Accordingly, a fiber optic gyroscope of the type encompassed by the present invention typically comprises an assembly which includes, among other elements, a coil of optical fiber mounted on and secured to a spool, and magnetic shielding enclosing the fibers. Such magnetic shielding may include the spool itself and a cover secured to the spool to form a magnetically protective housing or inner shield enclosing the coil. An outer or external shield is placed over this inner shield protective housing, with proper separation provided by a spacer ring to further enhance the magnetic shielding about the optical fiber coil. For some inertial navigation applications, a combined shielding factor (magnetic field attenuation) on the order of 104 may be required. In prior art constructions, the separate shields are typically are bonded together, through the intermediary of a spacer ring, to provide a fully assembled gyroscopic instrument. This assembly is then bolted to a stable support member, such as an inertial sensor block. Because the bonding of both the inner and outer shields is of a generally permanent nature, it is difficult to dismantle the assembly, such as for rework or repair. Such rework often occurs during early production, when it is needed to open the assembly, which may occur when minor failures need repair. Rework of such an assembly may result in damage or failure to the shield and/or its contents.
In the above-described construction, both the inner and outer shields are formed of a high magnetic permeability material, which is subjected to a stress annealing process to enhance and maximize its magnetic permeability. Such shields comprise a ferromagnetic material having high relative permeability (xcexc/xcexc0). Preferred high permeability materials include alloys, for example, of Carpenter High Permeability xe2x80x9c49xe2x80x9d(copyright) and Carpenter HyMu xe2x80x9c80xe2x80x9d(copyright) (trademarks of Carpenter Technology Corporation) whose compositions are respectively a 48% nickel-iron alloy and an unoriented 80% nickel-iron-molybdenum alloy.
The shields, when properly annealed, are generally malleable and, therefore, are subject to damage. They are strain and impact sensitive and, should the assembly be dropped, bent, dented, or even mishandled or slightly deformed, the magnetic permeability will be compromised and, thus, the magnetic shielding invariably will be deleteriously affected. Such mishandling or mishap can occur during the above-mentioned dismantling and disassembly, e.g., for repair and rework, when the inner and outer shields must be separated. Such dropping, bending, denting or mishandling can cause the assemblies, that is, the shields and their contained fiber optics and associated parts, to be destroyed and, therefore, they need replacement.
Another problem, to which the present invention provides a solution, relates to the connections between the respective parts of the inner and outer shields. The current practice of manufacturing the inner shield is to bond or weld, for example, the inner shield cover to an inner shield base or the optical fiber supporting spool. The outer shield assembly comprises upper and lower shield portions which must be bonded, welded or screwed together. Gaps at the joined portions deleteriously affect the magnetic flow path. These joining techniques also present other disadvantages and problems. For example, the bonding process is subjected to high labor costs and tight process controls. Both bonded and welded joints suffer from the difficulty, if not impossibility of rework, and the resulting scrapping of welded parts. Use of threaded parts requires the use of extremely tight, expensive machining tolerances, and the threads can gall, thus making rework difficult, if not impossible.
Further, the joint must be solid and robust, that is, free from movement between the portions over wide temperature excursions and vibration conditions, to prevent rattling during vibration which would harmfully affect operation of the gyroscope.
These and other problems are successfully addressed and overcome by the present invention by an elastic interference fit positioned between the cover and spool of the inner shield and between the upper and lower portions of the outer shield. In the preferred embodiment, this yieldable interference fit comprises a snap-fittable or spring-like biased joint engagement between the respective parts. Although they are subject to deformation, bending of the inner shell cover and the outer shield portions are within the elastic limits of the shield material, so that the deformation will not become permanent and affect their magnetic performance.
Several advantages are provided by this arrangement. Principally, deformation of the parts, but within their elastic limits, prevents loss of magnetic shielding over the course of repeated assembly and disassembly of the fiber optic gyroscope. Wide dimensional tolerance stamping with light touch-up machining is possible. Repair and rework is facilitated without damage to the shield or internal components. Labor costs for assembly and rework are reduced. Scrap is decreased, with corresponding savings in parts and materials.
Other aims and advantages, as well as a more complete understanding of the present invention, will appear from the following explanation of exemplary embodiments and the accompanying drawings thereof.